This application claims the benefit of Korean Patent Application No. 97-42828, filed Aug. 29, 1997, in the Korean Patent Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical pickup for a recordable and reproducible disk, and more particularly, to an optical pickup using laser light in which the wavelength of laser light which is used for reproducing information from a disk is different from that which is used for recording information on the disk.
2. Description of the Related Art
There have been proposed optical pickup technologies for freely recording and reproducing information on and from a disk, respectively, in order to heighten a utility of a recording medium such as an optical disk. In the case of a recordable and reproducible digital video disk (DVD) RAM whose specification is currently in progress, an optical pickup uses a laser light source which emits laser light having an output power of 30 mW and whose wavelength is approximately 650 nm. Since the laser light source emits the laser light to have a cross-section which is substantially elliptical, beam shaping is needed for the emitted laser light. The optical pickup also uses laser light of respectively different wavelengths for a reproduction mode for reproducing information from a recordable and reproducible disk and a recording mode for recording information on the optical disk, in order to obtain an optical power which is sufficient for recording information on the optical disk.
A conventional optical pickup performs beam shaping with respect to laser light of respectively different wavelengths which are used for a recording mode and reproduction mode.
The optical pickup shown in FIG. 1 includes a laser diode 11 which emits laser light having a cross-section which is substantially elliptical. The laser diode 11 emits reproduction laser light having a wavelength of approximately 650 nm in case of a reproduction mode and emits recording laser light having a wavelength of approximately 652.5 nm in case of a recording mode. A collimating lens 12 collimates the reproduction laser light and the recording laser light emitted from the laser diode 11. The collimated laser light is incident to a beam shaping prism 13. The beam shaping prism 13 beam-shapes the laser light incident from the collimating lens 12 so that a short beam diameter is substantially the same as a long beam diameter. The direction toward which beam shaping is performed is the same as that of a short beam diameter. In FIG. 1, the beam shaping direction is parallel with the surface of the paper sheet. The laser light beam-shaped by the beam shaping prism 13 is incident to a beam splitter 14. The beam splitter 14 transfers the laser light incident from the beam shaping prism 13 to a reflection mirror 15. The reflection mirror 15 reflects the laser light incident from the beam splitter 14 at a right angle. The laser light reflected by the reflection mirror 15 is focused on a disk 17 by an objective lens 16.
The laser light reflected from the disk 17 is reflected again by the reflection mirror 15 and then incident to the beam splitter 14. The beam splitter 14 reflects the laser light incident from the reflection mirror 15 and the laser light reflected by the beam splitter 14 is focused by a focusing lens 18 on a photodiode 19.
In the case when the optical pickup shown in FIG. 1 having the above optical system is used, the position of an optical spot of the reproduction laser light on the disk 17 is not consistent with that of the recording laser light. For example, if the wavelength of the reproduction laser light differs from that of the recording laser light by about 2.5 nm, an optical spot due to the recording laser light is formed on a position shifted by about 0.2 xcexcm from an optical spot due to the recording laser light along the direction toward which beam shaping is performed. The position movement of the optical spot is due to a chromatic aberration generated by the beam shaping in the optical system of the optical pickup. The chromatic aberration is generated by the beam shaping and variation of the wavelength by several nanometers when a reproduction mode is changed to a recording mode. The chromatic aberration changes within a short time so that an actuator cannot respond. Thus, the recording information is recorded in an undesired position on a disk along the size and direction of the chromatic aberration.
In a conventional optical pickup to solve the above problem, a laser diode is arranged to be slanted by a predetermined angle in the direction of the height of the optical pickup, that is, in the direction perpendicular to the surface of the disk, and a beam shaping prism performs beam shaping along the tangential direction of the disk. In the case of the conventional optical pickup, the optical spot formed on the disk does not shift in the radial direction of the disk although the wavelength has changed. However, even though the problem of the chromatic aberration can be solved by adjusting the arrangement of the laser diode, a new problem arises in that it is difficult to manufacture a compact optical pickup.
To solve the above problems, it is an object of the present invention to provide an optical pickup which can correct a chromatic aberration through dispersion in which the chromatic aberration has been generated by using laser light of respectively different wavelengths during recording and reproduction.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
To accomplish the above objects and advantages of the present invention, there is provided an optical pickup for a recordable and reproducible disk, including a laser light source to emit first laser light in case of a reproduction mode for reproducing information from the disk and to emit second laser light in case of a recording mode for recording information on the disk; a collimating lens to collimate the laser light emitted from the laser light source; an objective lens to focus the incident laser light on the disk; and chromatic aberration correction means having optical elements of respectively different dispersion powers, for beam-shaping and dispersing the laser light collimated by the collimating lens through the optical elements so that the chromatic aberration of an optical spot formed on the disk is corrected in which the chromatic aberration is generated by a difference between the wavelengths of the first and second laser light, and to transmit the beam-shaped and dispersed laser light to the objective lens.
The chromatic aberration correction means includes a first triangular prism having a first surface to which the laser light collimated by the collimating lens is incident and a second surface from which the laser light incident to the first surface emerges; and a second triangular prism having a first surface contacting the second surface of the first triangular prism. The first and second triangular prisms have geometrical structures and material properties for performing beam shaping and lateral chromatic aberration correction with respect to the first and second laser light, respectively, a third surface of the first triangular prism facing the vertex of the first triangular prism contacts the vertex of the second triangular prism, and an angle formed by the first surface and the third surface of the first triangular prism is an obtuse angle. These prisms correct the chromatic aberration through beam shaping and dispersion.